Nickeliferous lateritic ores are potentially the world's largest source of nickel and cobalt. In general, most deposits of nickel/cobalt laterites contain three major zones based on lithology, mineralogy and chemical composition. These three zones from the base to the surface, are the saprolite zone, the transition zone and the limonite zone, and generally sit atop weathered soil material. There is generally a large variation in total thickness of the nickeliferous lateritic ore deposit, as well as individual zone thickness.
The saprolite zone consists predominantly of “saprolitic serpentine minerals” and a large variety of nickel/magnesium silicate minerals. It contains between 0.5% wt and 4% wt nickel and a higher magnesium content, which is normally over 6% wt. The cobalt to nickel weight ratio of saprolite is normally less than 1:10.
The limonite zone, located on the top zone of nickeliferous lateritic ore body, contains nickel ranging from about 0.5% wt to 1.8% wt and consists of goethite-rich, magnetite-rich and/or hematite-rich ore, which is rich in iron, nickel and cobalt content. As it is the top zone, it is subjected to greater weathering which is characterised by a decrease in magnesium content and fine particle size, and an increase in the iron content. Therefore, it has lower magnesium content than saprolitic type ore.
Depending on weathering extent, limonitic ore contains dominantly fine and soft particles of goethite and/or hematite. Sometimes weathering has not been fully completed and magnetite rich sections are present. Alternatively, depending upon the climatic condition, there is formation of clay-type nickeliferous lateritic ores that contain nickel and/or cobalt containing iron/magnesium/aluminium silicates, such as smectite, nontronite and chlorite.
The transition zone is not normally well defined and is composed essentially of limonite and saprolite. It also commonly contains nickel in the range of from 1% wt to 3% wt. with co-existing cobalt ranging from 0.08% wt up to 0.3% wt.
Cobalt existence in zones of saprolite, limonite and transition is generally associated with asbolane, a mineral of hydrated manganese oxide. The cobalt value of a nickeliferous lateritic ore deposit is mostly recovered from the limonitic and transition zones.
Although nickeliferous lateritic ore deposits are exploitable with surface mining, they have historically been overlooked in favour of underground sulfide deposits as the nickel is readily concentrated by floatation techniques. This is despite the abundant source of nickel bearing laterite ore. Most nickeliferous lateritic ores are generally considered a lower grade of nickel bearing ore for whole ore refining, and more difficult to recover the nickel than from sulfide ores. However, as sulfide ore deposits begin to disappear, nickeliferous lateritic ore deposits are increasingly becoming an important source of nickel and cobalt.
The process for extracting nickel and cobalt from nickeliferous lateritic ores is generally confined to expensive and/or energy intensive methods. For example, it is known to directly smelt nickeliferous lateritic ore in furnace, which is quite an energy intensive process. In particular, the saprolitic component may be processed by pyrometallurgical means such as a rotary kiln and electric furnace (RKEF) process. The selected saprolite fraction should meet the specification of nickel grade and SiO2/MgO weight ratio settled by economics and slag-making conditions in smelter. In RKEF processing, the saprolitic fraction of a nickeliferous lateritic ore with reduced nickel and partially reduced iron is sent to an electric furnace for final reduction to metal as a ferronickel product. The RKEF process employs large rotary kilns to dry, calcine and reduce the nickel/iron bearing lateritic ores followed by a transfer to alternating current (AC) electric furnaces for smelting to ferronickel products. The slag is generally discarded. In some RKEF process, the slag is subjected to a further process named Metallic Nickel Recovery (MNR) from slag, which consist of physical separation steps (milling and electromagnetic separation) in order to increase nickel recovery as ferronickel.
The limonite component of a nickeliferous lateritic ore is generally processed in a hydrometallurgical process, such as a High Pressure Acid Leach (HPAL) process with concentrated sulfuric acid. This is a highly corrosive process requiring expensive and sophisticated equipment such as autoclaves, flash tanks etc. to perform the operation. Both the pyrometallurgical RKEF process and the hydrometallurgical HPAL process require a certain grade of nickel ore to make them economically viable.
Heap leaching is a method for economically extracting metals from ores that may not be suitable for either RKEF or HPAL processes. Generally, heap leaching simply involves stacking raw ore, taken directly from an ore deposit, into heaps that vary in height (4-7 m). The leaching solution (lixiviant) is irrigated upon the top of the heap to percolate down through the heap to produce a pregnant leach solution (PLS).
It has been found that the permeability of nickeliferous lateritic ore is largely controlled by the type of lithology, mineral morphology and particle size of the ore. Although the mineralogy of nickeliferous lateritic ore is rather complex and widely variable from deposit to deposit, there is some commonality or similarity of mineral morphology in the world-wide nickeliferous lateritic ore deposits. These morphological structures enhance permeability of solution, driven by blend ratios, and preserve physical stability of individual minerals.
Heap leaching of nickeliferous oxidic ore has been proposed in recovery processes for nickel and cobalt and is described, for example, in U.S. Pat. Nos. 5,571,308 and 6,312,500, both in the name of BHP Minerals International, Inc.
U.S. Pat. No. 5,571,308 describes a process for heap leaching of high magnesium containing nickeliferous lateritic ore such as saprolite. The patent points out that the fine saprolite exhibits poor permeability and a solution to this, pelletisation or agglomeration of the ore is necessary to ensure distribution of the lixiviant through the heap.
U.S. Pat. No. 6,312,500 also describes a counter-current process for heap leaching of laterites to recover nickel and is particularly directed to ores that have a significant clay component (greater than 10% by wt.). This process includes sizing of the ore where necessary, forming agglomerates by contacting the ore with a lixiviant, and agglomerating. The pellets are formed into a heap and leached with sulfuric acid solution to extract the metal values. In addition to fresh water, the sulfuric acid fortified seawater, recycled solutions such as raffinates or process water may be used as a lixiviant.
U.S. Pat. No. 7,597,738, in the name of BHP Billiton SSM Development Pty. Ltd. describes a process for the production of ferronickel with mixed iron/nickel hydroxide precipitate from a heap leach/ion exchange (IX) process. The pregnant leach solution (PLS) from the heap leach process, having a pH in the range of 1.5-2.0, is treated with an ion exchange resin with functional group of bis-picolylamine to separate nickel from impurities such as ferrous ions Fe2+, Al, Cr, Mg and Mn with the exception of ferric ions. The preferred resin is Dowex M4195™, which has considerable affinity for Ni and ferric ions. As the PLS has a high Fe/Ni concentration ratio, the effective capacity of resin to load nickel is reduced, given that it will also co-load considerable quantities of ferric iron. This leads to the need for high investment in IX resin and equipment to maintain a given nickel production capacity. In addition, the high iron content in the IX eluate increases the reagent consumption to produce a mixed iron/nickel hydroxide precipitate (MHP) and decreases the nickel content in the MHP and therefore, the final product of ferronickel.
International application PCT/AU2006/000606 (in the name of BHP Billiton SSM Technology Pty Ltd) also describes a process where nickeliferous oxidic ore is heap leached using an acid supplemented hypersaline water as the lixiviant.
Heap leaching of laterites by sulfuric acid at ambient temperatures is also reported in various publications for example, by S. Agatzini, in the paper published in Hydrometallurgy 1994, Institution of Mining and Metallurgy, London 1994, page 193-208 titled “Heap Leaching of Poor Laterites”. 
Heap leaching nickeliferous lateritic ores offers the promise of a low capital cost process and rapid ramping-up, eliminating the need for expensive and high maintenance high pressure equipment required for conventional high pressure acid leach processes. Generally, in a heap leach process of nickeliferous lateritic ores, a relatively stronger acidic lixiviant than conventional copper heap leach is used to liberate both the cobalt and nickel from the cobalt and nickel containing ores.
Ashok D Dalvi et al (“The Past and Future of Nickel Laterite”, Inco Limited, PDAC 2004 International Convention, Trade Show & Inventor Exchange, Mar. 7-10, 2004) discloses that the RKEF process and the HPAL process become relatively unattractive if the feed nickel grade falls below 2% for the RKEF process and 1.4% for the HPAL process. In addition, in order to prevent corrosion of furnace lining and to control the amount of slag “super heat” required to enable tapping of the ferronickel in the RKEF process, the SiO2/MgO weight ratio of feed ore should be either greater than 2.5 or less than 2. As a result of these two constraints, relatively large quantities of nickeliferous lateritic ore becomes unsuitable for RKEF processing and is either stockpiled as so called “low grade ore” or kept unexploited. There is estimated to be ten million tons of ore that have been stockpiled in recent history, either with a nickel grade of between less than about 1.5% wt or with unsuitable SiO2/MgO wt. ratio and not processed because of its lack of suitability for either RKEF, or indeed high pressure leach processing.
It is a desired feature of the present invention to develop a comprehensive process where the nickeliferous lateritic ore can be commercially utilised in a conventional RKEF process, particularly so called low grade nickeliferous lateritic ore.
It is a further desired feature of the present invention to develop a process where a product from a heap leach process can be integrated into a conventional RKEF process.
It is a further desired feature to develop a process to produce a high grade nickel product that may be suitable for further processing to produce value added nickel products such as nickel cathodes, nickel nuggets or battery chemicals.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that document or matter was known or that the information it contained was part of the common general knowledge as at the priority date of any of the claims.